This invention relates to a process for the production of ethanol from cereals wherein the fermentation is continuously carried out with clean musts, i.e. with sugar solutions provided by enzyme saccharification from an acidified hydrolyzed product of cereals from which the insoluble solids have been separated by filtration.
The invention relates also to a continuous cycle apparatus to carry out said process.
The ethanol is provided both in a synthetic way from ethylene and by fermentation from sugar solutions. Due to the increase of ethylene cost the production of alcohols by fermentation has assumed a greater importance and industrial interest.
The fermentation is carried out under anaerobic conditions and with a non-continuous process or, which is less usual, with a continuous process by using sugar solutions with an usual concentration of fermentable sugars of about 15% in weight so as to provide maximum concentrations of 8-10% in volume once the sugars are consumed.
When the transformation of sugars into alcohols is completed, the wine is usually centrifugated to separate the yeast and is led to a distillation apparatus for the recovery of the alcohol.
The separation of the yeast cannot be carried out when turbid solutions are treated.
The yeast is recycled to the fermentation unit of the continuous fermentation plants.
The waste waters, i.e. the residue from the distillation of the fermented alcohol must have a high pollution load so that a preliminary treatment is necessary before their wasting.
The most commonly used treatment consists of a concentration by evaporation in a multiple-effect system until a concentration of about 60% of solid content is reached; the concentrated residues from the distillation of the fermented alcohol musts have a good bargain as fodder but a high vapour quantity is requested for their concentration.
The starting materials for the production of ethanol in a fermentative way can be sugar substances like sugar cane or sugarbeet molasses, sugar cane, sugarbeet, sugar sorghum and fruit, starchy substances like cereal, cassava and potatoes or finally cellulose substances like agricultural residues, wood and urban waste.
In Europe and the United States the trend in the last years is to provide ethanol from the surplus cereal production and from molasses; in tropical countries and first of all in Brazil the most used starting material is the sugar cane or the molasses.
The present status of art provides knowledges and applications on a sound basis as far as the continuous fermentation of clean solutions mainly deriving from sugar starting materials is concerned, while as for the production of ethanol from cereals (maize, corn, sorghum, a.s.o.) the use of non-continuous fermentation system is commonly provided without preliminary separation of the solid content, thus giving rise to the following disadvantages:
1. very long fermentation times (72 hours or more); PA0 2. impossibility of separating and recycling the yeast with the consequence of a low efficiency; PA0 3. operative difficulties in fermentation units of large dimensions due to the presence of solids suspended in the must; PA0 4. problems of eliminating the must in the fractionating columns still due to the presence of high percentages of suspended solids; PA0 5. high energy consumption and mechanical problems in drying the residue material. PA0 1. The fermentation of solutions containing solid suspensions has structural and operative problems due to the necessity of maintaining an uniform stirring in the fermentation unit itself in order to prevent solids from being deposited; PA0 2. The fermentation of concentrated sugar substrates requires the use of particular cultures of yeasts of the Saccharomyces type; PA0 3. The obtained residues from the distillation of the fermented alcohol musts cannot be concentrated in a multiple-effect evaporation system due to the high concentration of insoluble solids and are then dried with a low thermal efficiency; PA0 4. The treatment of dirty wine in the stripping column has non-negligible dirt problems. PA0 1. The separation of the insoluble solids before the fermentation phase allows the knowledge about the continuous fermentation with recycle of the yeast to be applied to clean musts (molasses, cane syrup, a.s.o.). PA0 2. The continuous fermentation of clean musts requires reduced fermentation times of the order of 30--35 hours due to the increase of concentration of the microorganisms by the recycle of the yeast; PA0 3. The fermentation of clean solutions at the usual sugar concentrations of 13-15% in weight allows commercial yeast to be used without need of preparing particular cultures; PA0 4. The recycle of the yeast further allows the fermentation yield to be increased, thus avoiding to produce huge quantities of yeast at the beginning of any fermentation and during the same; PA0 5. The elimination of suspended solids before of the fermentation allows the construction of fermentation systems of large dimensions to be simplified, and to make easier the operation thereof; PA0 6. The distillation column for clean wines is not affected with problems of dirt causing frequently stops for maintenance; PA0 7. The obtained residues from the distillation of the fermentated alcohol musts are free from insoluble solids and can be then concentrated in a multiple-effect evaporation system until a concentration of 60% in solid content is reached, thus achieving a strong energy saving with respect to a drying system. The filtering panels provided in the separation phase of the insoluble solids and destined for the zootechnic use are dehydrated by means of mechanical compression in order to reduce the consumption of vapour for the drying process.
The reason for such a situation is bound to the present state of the industry of producing ethanol from cereals, the aim of which has always been the production of alcoholic drinks without dealing, except for the recent times, with the problems relative to the production of ethanol in large quantities with both thermic and transforming efficiency.
A process (Biostill by Alfa-Laval) providing the continuous fermentation of concentrated sugar substrates and using as starting material molasses, cane, syrup and cereals has been thrown on the market in recent times.
Such a process causes the fermentation of a concentrated sugar solution provided, in case of cereals, by hydrolysis and saccharification of the starch and containing in suspension all insoluble solids present in the starting material.
In order to cause the starch to be separated, the fibers and the other insoluble solids are eliminated from the fermented wine by means of a stationary screen; the so clarified wine is then led to the centrifugal separator of the yeast.
The yeast is recycled for the continuous fermentation thereof and the limpid wine is led to the upper section of a stripping column for eliminating the ethanol (from the upper part); the exhausted wine is led back to the fermentation unit.
The insoluble solids separated from the screen are also led to the stripping column for the recovery of the residual ethanol.
The continuous transfer of the fermented wine to the stripping column, thus eliminating the ethanol, allows the fermentation to be continued with concentrated substrates, even if the content of ethanol is lower than 8% in volume in the fermentation unit; too high concentration of ethanol prevents the yeast to be active.
The residues from the distillation of the fermented alcohol musts outflowing from the bottom of the stripping column and containing all non-fermentable soluble solids and the insoluble substances are led directly to the drying system.
The Biostill process, however, has the following disadvantages:
This invention seeks to avoid all above mentioned problems by providing a process for the production of ethanol from integral cereals wherein the continuous fermentation take place on clean saccharified musts, i.e. (not concentrated) sugar solutions obtained by enzyme saccharification of an acidified hydrolyzed product obtained by an enzyme treatment of cereals meal after having separated by filtration the insoluble solids contained in said hydrolyzed product.
This is possible according to the invention with a process comprising the steps of:
screening a hydrolyzed product obtained by an enzyme treatment of a cereal meal, thus providing a limpid phase and a turbid phase; PA1 filtering by washing in a horizontal band filter under vacuum the turbid phase, thus providing a filtration panel and a filtrate; PA1 subjecting the filtration panel to a further mechanical squeezing, thus providing a dehydrated panel and a filtrate; PA1 leading the turbid filtrates obtained in the filtration phases to the stationary screen, thus providing a limpid filtrate; PA1 subjecting the limpid filtrate and the limpid hydrolyzed product to the enzyme saccharification treatment, thus providing a solution of fermentable sugars; PA1 fermenting the sugar solution in a set of atmospheric fermentation units with outer cooling and recycling system; PA1 centrifugating the wine obtained from the fermentation, separating the microorganism and leading them to the first fermentation unit after acidification, thus providing a limpid alcohol solution which is the final product to be treated in the distillation unit.
The following advantages are achieved from such a process:
It should be noted that there are other industrial applications providing the separation of the suspended solids from the saccharified must and the subsequent continuous fermentation with the drawback that the separation should be carried out at not so very high temperatures (about 60.degree. C.) and in the presence of fermentable sugars.
According to the present invention the suspended solids are separated at high temperatures (about 85.degree. C.) and in the presence of non-fermentable dextrines, thus reducing the danger of infection by microorganisms and making the separation easier due to the high temperature.